I. Field of the Invention
The current invention relates to wireless data communication. More particularly, the present invention relates to a novel and improved method and apparatus for high rate packet data transmission in a wireless communication system.
II. Description of the Related Art
A modern day communication system is required to support a variety of applications. One such communication system is a code division multiple access (CDMA) system which conforms to the xe2x80x9cTIA/EIA/IS-95 Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System,xe2x80x9d hereinafter referred to as the IS-95 standard. The CDMA system allows for voice and data communications between users over a terrestrial link. The use of CDMA techniques in a multiple access communication system is disclosed in U.S. Pat. No. 4,901,307, entitled xe2x80x9cSPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS,xe2x80x9d and U.S. Pat. No. 5,103,459, entitled xe2x80x9cSYSTEM AND METHOD FOR GENERATING WAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEM,xe2x80x9d both assigned to the assignee of the present invention and incorporated by reference herein.
In this specification, base station refers to the hardware with which the subscriber stations communicate. Cell refers to the hardware or the geographic coverage area, depending on the context in which the term is used. A sector is a partition of a cell. Because a sector of a CDMA system has the attributes of a cell, the teachings described in terms of cells are readily extended to sectors.
In a CDMA system, communications between users are conducted through one or more base stations. A first user on one subscriber station communicates to a second user on a second subscriber station by transmitting data on the reverse link to a base station. The base station receives the data and can route the data to another base station. The data is transmitted on the forward link of the same base station, or a second base station, to the second subscriber station. The forward link refers to transmission from the base station to a subscriber station and the reverse link refers to transmission from the subscriber station to a base station. In IS-95 systems, the forward link and the reverse link are allocated separate frequencies.
The subscriber station communicates with at least one base station during a communication. CDMA subscriber stations are capable of communicating with multiple base stations simultaneously during soft handoff. Soft handoff is the process of establishing a link with a new base station before breaking the link with the previous base station. Soft handoff minimizes the probability of dropped calls. The method and system for providing a communication with a subscriber station through more than one base station during the soft handoff process are disclosed in U.S. Pat. No. 5,267,261, entitled xe2x80x9cMOBILE ASSISTED SOFT HANDOFF IN A CDMA CELLULAR TELEPHONE SYSTEM,xe2x80x9d assigned to the assignee of the present invention and incorporated by reference herein. Softer handoff is the process whereby the communication occurs over multiple sectors which are serviced by the same base station. The process of softer handoff is described in detail in copending U.S. Pat. No. 5,625,876, entitled xe2x80x9cMETHOD AND APPARATUS FOR PERFORMING HANDOFF BETWEEN SECTORS OF A COMMON BASE STATION,xe2x80x9d assigned to the assignee of the present invention and incorporated by reference herein
Given the growing demand for wireless data applications, the need for very efficient wireless data communication systems has become increasingly significant. The IS-95 standard is capable of transmitting traffic data and voice data over the forward and reverse links. A method for transmitting traffic data in code channel frames of fixed size is described in detail in U.S. Pat. No. 5,504,773, entitled xe2x80x9cMETHOD AND APPARATUS FOR THE FORMATTING OF DATA FOR TRANSMISSION,xe2x80x9d assigned to the assignee of the present invention and incorporated by reference herein. In accordance with the IS-95 standard, the traffic data or voice data is partitioned into code channel frames which are 20 msec wide with data rates as high as 14.4 Kbps.
A significant difference between voice services and data services is the fact that the former imposes stringent and fixed delay requirements. Typically, the overall one-way delay of speech frames must be less than 100 msec. In contrast, the data delay can become a variable parameter used to optimize the efficiency of the data communication system. Specifically, more efficient error correcting coding techniques which require significantly larger delays than those that can be tolerated by voice services can be utilized. An exemplary efficient coding scheme for data is disclosed in U.S. Pat. No. 5,933,462, entitled xe2x80x9cSOFT DECISION OUTPUT DECODER FOR DECODING CONVOLUTIONALLY ENCODED CODEWORDS,xe2x80x9d assigned to the assignee of the present invention and incorporated by reference herein.
Another significant difference between voice services and data services is that the former requires a fixed and common grade of service (GOS) for all users. Typically, for digital systems providing voice services, this translates into a fixed and equal transmission rate for all users and a maximum tolerable value for the error rates of the speech frames. In contrast, for data services, the GOS can be different from user to user and can be a parameter optimized to increase the overall efficiency of the data communication system. The GOS of a data communication system is typically defined as the total delay incurred in the transfer of a predetermined amount of data, hereinafter referred to as a data packet.
Yet another significant difference between voice services and data services is that the former requires a reliable communication link which, in the exemplary CDMA communication system, is provided by soft handoff. Soft handoff results in redundant transmission s from two or more base stations to improve reliability. However, this additional reliability is not required for data transmission because the data packets received in error can be retransmitted. For data services, the transmit power used to support soft handoff can be more efficiently used for transmitting additional data.
The parameters which measure the quality and effectiveness of a data communication system are the transmission delay required to transfer a data packet and the average throughput rate of the system. Transmission delay does not have the same impact in data communication as it does for voice communication, but it is an important metric for measuring the quality of the data communication system. The average throughput rate is a measure of the efficiency of the data transmission capability of the communication system.
It is well known that in cellular systems the carrier-to-interference ratio C/I of any given user is a function of the location of the user within the coverage area. In order to maintain a given level of service, TDMA and FDMA systems resort to frequency reuse techniques, i.e. not all frequency channels and/or time slots are used in each base station. In a CDMA system, the same frequency allocation is reused in every cell of the system, thereby improving the overall efficiency. The C/I that any given user""s subscriber station determines the information rate that can be supported for this particular link from the base station to the user""s subscriber station. Given the specific modulation and error correction method used for the transmission, which the present invention seeks to optimize for data transmissions, a given level of performance is achieved at a corresponding level of C/I. For idealized cellular system with hexagonal cell layouts and utilizing a common frequency in every cell, the distribution of C/I achieved within the idealized cells can be calculated. An exemplary system for transmitting high rate digital data in a wireless communication system is disclosed in copending U.S. patent application Ser. No. 08/963,386, entitled xe2x80x9cMETHOD AND APPARATUS FOR HIGHER RATE PACKET DATA TRANSMISSION,xe2x80x9d (hereafter the ""386 application), now U.S. Pat. No. 6,574,211, issued on Jun. 3, 2003 to Padovani et al., assigned to the assignee of the present application and incorporated by reference herein.
It is also well known that much of the signal interference in a loaded CDMA system is caused by transmitters belonging to the same CDMA system. In an effort to increase capacity, cells are often divided into sectors or smaller cells operating at lower power, but such methods are costly and difficult to apply in areas having widely varying signal propagation properties. The data communication system of the present invention provides a way of decreasing the mutual interference between elements in the system without requiring a large number of small cells.
The present invention is a novel and improved method and apparatus for high rate packet data transmission in a CDMA system. The present invention improves the efficiency of a CDMA system by providing a means of providing a strong forward link signal to a destination subscriber station while causing minimal interference to other subscriber stations.
The present invention provides an alternate approach to maximizing capacity in a high-data-rate wireless system by adapting beamforming techniques for use in terrestrial wireless applications. In accordance with the present invention, a cellular system with multiple transmit antennas at each base station is described. From each base station, the same signal, but with each having different relative phase shifts and power levels, is transmitted from each antenna. In order to maximize the carrier-to-interference ratio (C/I) of the intended receiver of the signal (usually a single subscriber station) the phases of the signals being transmitted from each of the transmit antennas must be set appropriately.
One method of maximizing C/I at the subscriber station is by determining the channel impulse response from each of the serving base station""s transmit antennas to the subscriber station. The serving base station requires knowledge of the phase and gain of each signal received at the subscriber station antenna from each transmit antenna of the serving base station. Therefore, a scheme must be devised to allow the subscriber station to estimate phase and gain of the signal received from each of the transmit antennas. One method is to send a reference signal having characteristics known by both the transmitter and the receiver on each of the transmit antennas. In an exemplary embodiment of the present invention, a reference signal burst is sent from each antenna of the base station, allowing the subscriber station to estimate the channel impulse response corresponding to each of the transmit antennas separately. The reference signal bursts may be separated either by transmitting the bursts through one antenna at a time, or by using a different code space for each antenna, such as a different Walsh code for each antenna.
The base station may alternatively send channel impulse response reference signals continuously on each transmit antenna, but using a reference signal having a different structure for each antenna. The subscriber stations may detect the different references separately and estimate the channel impulse response corresponding to each transmit antenna. When there are multiple receive antennas at the subscriber station, then the subscriber station must estimate the channel impulse response corresponding to each transmit antenna-receive antenna pair.
The subscriber station transmits a signal indicative of the estimated channel impulse responses corresponding to each transmit antenna-receive antenna pair to the base station on the reverse link. Once the channel impulse response of each transmit antenna-receive antenna pair is known, the base station may then optimally form a beam toward each subscriber station.
An alternative method of adjusting the signals sent from the transmit antennas is based on sending signal quality feedback other than channel impulse response from the subscriber station to the base station. For example, the subscriber station may measure the C/I it receives and send to the base station a signal indicative of the estimated received C/I value. The base station may then adjust the phase of the signal transmitted on one or all of its transmit antennas. The subscriber station then makes a new estimate of the received C/I and sends that estimate to the base station. The base station compares the new C/I with the old C/I. If the C/I increased, the base station further adjusts the phases of the transmit signals in the same direction as before in order to further increase the C/I at the subscriber station. If, however, the new C/I is lower than the old C/I, the base station adjusts the transmit phases in the opposite direction. Different algorithms may be used to update the transmit signal phase and gains on the different antennas based on signal quality feedback from the subscriber stations.
Any signal quality metric that is based on the estimated C/I may be used by the subscriber station as feedback to the base station. In the exemplary high-data-rate wireless communication system described in the ""386 application, the subscriber station determines a data rate at which it can successfully receive packets based on its estimated C/I. The data rate, instead of the C/I measurement, is sent to the base station in the form of a data rate control (DRC) signal. In the exemplary system, DRC information is embedded in the reverse link signal sent by the subscriber station. The base station may then use the changes in the DRC signal to determine whether it is changing the phase of the transmit signal in the right direction. Once the phases corresponding to the maximum data rate control (maximum DRC) signal from a subscriber station are found, the base station uses those phases for all transmissions to that particular subscriber station. Usually, the base station must schedule packets to be transmitted to multiple subscriber stations. In this situation, a scheduling algorithm is devised which determines the order in which packets are transmitted on the forward link to different subscriber stations. Once the scheduler decides which subscriber station to serve, the base station uses the phases corresponding to the maximum DRC to transmit signals to that subscriber station.
In the exemplary system described in the ""386 application, the C/I and DRC information measured at a subscriber station is based on comparing a signal from the serving base station to interference from all other base stations. The interference caused by other base stations at any one time depends on the phases of the signals transmitted by those base stations. Suppose that during the time slot within which the subscriber station is scheduled to receive data from a first base station, a second base station changes its transmit phases. This may increase the interference level at the subscriber station under consideration, reducing the reliability of receiving the signal transmitted by the first base station to an unacceptable level, resulting in an increase of the packet error rate at the subscriber station.
One way to solve the preceding problem is for each base station to allocate an additional signal burst (referred to herein as a DRC reference burst), which is sent using the transmit phases that the base station intends to use a predetermined number of slots in the future. The subscriber stations may then compute appropriate future forward link data rates using the DRC reference burst. This way the subscriber stations will estimate future DRC""s knowing what the interference levels will be. Therefore, during each time slot two types of signal bursts will be sent, data pilot bursts for demodulating the data in the current slot and DRC reference bursts for estimating the DRC of two slots from now. Note that the data pilot is being sent using the same transmit phases as are used to send the data in the current slot.
In the proposed Third Generation CDMA systems, the signals are modulated using a quaternary phase shift keyed (QPSK) modulation. In order to balance the load on the in-phase (I) and quadrature phase (Q) components of the QPSK signals, a method of complex PN spreading is employed. Complex PN spreading is described in U.S. patent application Ser. No. 08/856,428, entitled xe2x80x9cREDUCED PEAK-TO-AVERAGE TRANSMIT POWER HIGH DATA RATE IN A CDMA WIRELESS COMMUNICATION SYSTEM,xe2x80x9d assigned to the assignee of the present invention and incorporated by reference herein.
A method and apparatus for demodulating signals from different base stations in soft handoff and for improved signal estimation based on multipath reception is described in detail in U.S. Pat. No. 5,109,390, entitled xe2x80x9cDIVERSITY RECEIVER IN A CDMA CELLULAR TELECOMMUNICATION SYSTEM,xe2x80x9d assigned to the assignee of the present invention and incorporated by reference herein.
A method and apparatus for performing search and acquisition in a CDMA communication system is disclosed in detail in U.S. Pat. Nos. 5,644,591 and 5,805,648 entitled xe2x80x9cMETHOD AND APPARATUS FOR PERFORMING SEARCH ACQUISITION IN A CDMA COMMUNICATIONS SYSTEM,xe2x80x9d both assigned to the assignee of the present invention and incorporated by reference herein.